It is doable as an older refit of the UM-R60. If you must have the same ammo I suppose one heat sink could be swapped for more ammo.
Urbanmech UM-R60
Mass: 30 tons
Tech Base: Inner Sphere
Chassis Config: Biped
Rules Level: Experimental Tech
Era: Clan Invasion
Tech Rating/Era Availability: E/X-X-F
Production Year: 3070
Cost: 1,756,625 C-Bills
Battle Value: 652
Chassis: Unknown Standard
Power Plant: Unknown 60 Fusion Engine
Walking Speed: 21.6 km/h
Maximum Speed: 32.4 km/h
Jump Jets: Unknown
Jump Capacity: 60 meters
Armor: Unknown Ferro-Fibrous w/ CASE
Armament:
1 Thunderbolt-15
1 Small Laser
Manufacturer: Unknown
Primary Factory: Unknown
Communications System: Unknown
Targeting and Tracking System: Unknown
================================================================================
Equipment Type Rating Mass
--------------------------------------------------------------------------------
Internal Structure: Standard 51 points 3.00
Engine: Fusion Engine 60 1.50
Walking MP: 2
Running MP: 3
Jumping MP: 2 Standard
Jump Jet Locations: 2 CT 1.00
Heat Sinks: Single Heat Sink 11 1.00
Heat Sink Locations: 1 HD, 2 LT, 2 RT, 2 LL, 2 RL
Gyro: Standard 1.00
Cockpit: Standard 3.00
Actuators: L: SH+UA R: SH+UA
Armor: Ferro-Fibrous AV - 96 5.50
Armor Locations: 4 LT, 3 RT, 4 LA, 3 RA
CASE Locations: 1 RT 0.50
Internal Armor
Structure Factor
Head 3 9
Center Torso 10 11
Center Torso (rear) 8
L/R Torso 7 8
L/R Torso (rear) 4
L/R Arm 5 10
L/R Leg 7 12
================================================================================
Equipment Location Heat Critical Mass
--------------------------------------------------------------------------------
Thunderbolt-15 RT 7 3 11.00
Small Laser LA 1 1 0.50
@Thunderbolt-15 (8) RT - 2 2.00
Free Critical Slots: 19